How DVDs and CDs Store Data

Reading and Writing a Digital Versatile Disk or Compact Disk

Mar 12, 2009

Digital Versatile Disks (DVD) and Compact Disks (CD) are widely used storage devices. The recordable type use Phase Change Materials to store data.

The article How Binary Numbers are Used to Store Data describes how numbers, text, and graphics are stored using either a "1" or a "0". This article describes how each "1" or "0" may be written to or read from a DVD or CD. There are two main methods used, depending on the volume of disks to be recorded, and this article concentrates on the chemistry of Phase Change Materials.

CD and DVD Data Storage - Mechanical Processes

Large-scale production of DVDs or CDs uses a mechanical process called Precision Moulding, where each "bit" (short for "binary digit") is either an "up" (bump) or a "down" (pit). Recordable DVDs or CDs used by most consumers use Phase Change Materials to define each "bit". See Figure 1 for a simplified diagram of a DVD track.

CD and DVD Data Storage - Why Use Phase Change Materials?

Phase Change Materials are used in DVDs and CDs because they can be made into reflective or non-reflective materials. The states "reflective" and "non-reflective" can be designated as a "0" or "1", in a recordable DVD or CD. Certain materials can be changed from reflective to non-reflective states, depending on their structure.

CD and DVD Data Storage - What Makes Phase Change Materials Reflective?

Phase Change Materials are more transparent when they are crystalline. Crystalline materials have an ordered, regular structure. It is usually the case that larger, more regular crystals are obtained by using a slower cooling rate, and so it is with Phase Change Materials. The rate of cooling depends on the difference between the surrounding temperature and the material being cooled, so to achieve a slow cooling rate, the initial temperature is lower than the melting point of the material. This gives a crystalline structure to the Phase Change Material. To achieve a faster cooling rate, the material is heated to a temperature that is higher than the melting point. This leads to a non-crystalline or amorphous structure. Figure 2 shows how the amorphous and crystalline states represent either "1" or "0".

CD and DVD Data Storage - Where are the Phase Change Materials in a CD or DVD?

Figure 3 shows this in simplified form. The DVD or CD has a Phase Change Material in the middle, surrounded by dielectric layers. A reflective layer is above the Phase Change Material, so that the light produced by the reading device can be reflected back if it passes through the crystalline material. It is then picked up by a detector. The other layers are there for protection.

CD and DVD Data Storage - What Chemicals are Used?

Materials are chosen that can change quickly between states, so that data can be recorded quickly. The two main families of Phase Change materials are Germanium Antimony Tellurium (GeSbTe) alloys, and Silver Indium Antimony Tellurium (AgInSbTe) alloys.

CD and DVD Data Storage - Summary

Digital Versatile Disks encode data using the binary number system. At the most basic level, the various types of data - numerical, text, images and music - can all be represented using only the digits "0" and "1". These "bits" are arranged in groups of eight, called bytes. Recordable DVD and CD use Phase Change Materials to store data, and these work because their reflective properties can be changed depending on how quickly they are cooled after heating.

CD and DVD Data Storage - References

The copyright of the article How DVDs and CDs Store Data in Everyday Chemistry is owned by Martin Bell. Permission to republish How DVDs and CDs Store Data in print or online must be granted by the author in writing.